1. Field of the Invention
This invention relates generally to telecommunication systems, and, more particularly, to wireless telecommunication systems.
2. Description of the Related Art
Wireless authentication systems typically include one or more base stations (sometimes referred to as node-Bs) and a plurality of mobile units. The mobile units may include cellular telephones or other user equipment, such as desktop personal computers, laptop computers, personal data assistants, pagers, and the like. The base stations and the mobile units may exchange voice and/or data information over an air interface. To transmit the voice and/or data information, the base station or mobile unit encodes the information according to one or more wireless telecommunication protocols such as Universal Mobile Telephone System (UMTS) protocol, a Global System for Mobile communications (GSM) protocol, a Code Division Multiple Access (CDMA, CDMA 2000) protocol, and the like. The encoded information is then used to modulate an electromagnetic wave, which is transmitted across the air interface. For example, the carrier electromagnetic wave may be a 1 GHz radio frequency carrier wave.
The modulated carrier wave transmitted by a mobile unit may be relatively weak when it is received at a base station. The modulated carrier wave may also be obscured by various sources of noise, including noise introduced by the mobile unit and environmental noise. Accordingly, one or more antennas at the base station are typically coupled to one or more filters, which are tuned to the frequency of the carrier wave. For example, a base station antenna may be coupled to a 1 GHz radio frequency filter to reduce various noise components and thereby improve the selectivity of the base station to 1 GHz radio frequency carrier waves transmitted by the mobile units associated with the base station.
The physical characteristics of conventional filters used in wireless telecommunication systems are determined by the wavelength of the carrier wave. For example, a 1 GHz radio frequency carrier wave has a wavelength of approximately 30 cm. Thus, the height of a quarter-wave resonator in a conventional duplex filter that is tuned to receive the 1 GHz carrier wave must be approximately 7.5 cm. Other wireless telecommunication systems may use lower frequencies, such as 450 MHz CDMA systems, which results in proportionately larger filters. At least in part to achieve a desired Q-factor, the duplex filter typically includes between 12 and 14 resonators. Thus, the form factor of a conventional duplex filter can be quite large. For example, the dimensions of a duplex filter tuned to 1 GHz may be 50 cm×7.5 cm×6 cm.
Conventional duplex filters are formed of metals that may expand and contract as the ambient temperature increases and decreases over time. The expansion and/or contraction of portions of the conventional duplex filter may detune the filter because the wavelength selected by the duplex filter is proportional to the physical dimensions of the filter. In order to maintain filter tuning during thermal deformation, conventional filters may be tuned over a small range of frequencies, typically approximately 5%. However, tuning the conventional filter results in power losses that may reduce the selectivity of the antenna and may also degrade the Q-factor of the filter. Filter losses may be about 0.5 dB and tuning elements can increase the losses to about 3.0 dB. Base station selectivity may thus be reduced by a similar amount, which may also reduce the radius of the cell served by the base station. Furthermore, conventional filters may only be used with a single carrier wave frequency because the power losses and/or degradation of the Q-factor typically prevent re-tuning the conventional filter to a different frequency band.
Conventional duplex filters are fabricated from relatively heavy materials, at least in part to reduce the deformation that may be caused by temperature changes and to reduce the corresponding detuning of the filter. For example, a conventional duplex filter may weigh as much as 10 kg. The large mass of the conventional duplex filter may substantially increase the mass of the duplex filters associated with each base station. For example, a base station that serves mobile units in three sectors may include two antennas for each sector. Each antenna requires at least one associated duplex filter, resulting in a total of at least six duplex filters, having a total weight of about 60 kg. Moreover, in Multiple Input Multiple Output (MIMO) systems, the total number of antennas, and therefore the total number of duplex filters, generally increases in proportion to the number of input and/or output channels of the MIMO system. For example, a typical MIMO antenna system uses four antennas per sector, which results in 12 duplex filters weighing approximately 120 kg.
Fabricating some components of conventional duplex filters from materials having a relatively high dielectric constant may reduce the dimensions of some filters. For example, ceramic materials have dielectric constant of ε. If a portion of the filter is fashioned from a ceramic material, the height of the filter may be reduced by a factor of approximately √{square root over (ε)}. Thus, if a ceramic material has a dielectric constant of ε=4, the height of the filter may be reduced by a factor of approximately 2. However, the physical dimensions of conventional filters fabricated from ceramic materials are still tied to the wavelength of the carrier wave. The mass of the conventional duplex filter may also be reduced by incorporating ceramic materials and reducing the physical dimensions of the conventional filter. However, the reductions in mass are modest because the physical dimensions of the conventional filter are still tied to the wavelength of the carrier wave.
The present invention is directed to addressing the effects of one or more of the problems set forth above.